The simplest fault detector employed in electrical apparatus is the simple impedance relay which continuously calculates the ratio of voltage across the apparatus to current through it. When a fault occurs, the impedance of the apparatus changes and the fault detector registers a fault. The problem with this protection system is that it is not very sensitive and when applied to air core reactors simply cannot detect the small faults which can occur in these devices.
Differential protection systems have been applied very successfully to iron cored electrical apparatus like generators, transformers and iron cored reactors. Current transformers at either end of the apparatus compare the currents entering and leaving the winding. When a ground fault occurs, the current leaving is not equal to the current entering the winding and the detector registers a fault. Winding to ground faults cannot easily occur on an air cored winding and therefore the system is not useful for air cored reactors.
In another known differential relaying system, useful to protect electrical apparatus in which the winding comprises two identical halves connected in parallel, current transformers continuously compare the currents in two halves of the winding and when a fault occurs in either winding the resulting imbalance in currents produces a detector signal which signifies that a fault has occurred. The difficulty with this scheme when applied to any air core reactor is that it is unable to detect a turn to turn fault in many reactors, particularly in those reactors which consist of a very large number of windings in parallel.
In a variant of the preceeding, a single detector is used to detect a fault in any one phase of a three phase system. It works in essentially the same manner as the preceeding system, but in this arrangement a single detector is able to detect when a fault occurs in any one of the three windings of a three phase device. When applied to air cored reactors, the system suffers from the same limitations as the preceeding system, namely that it is not sensitive enough to detect turn to turn faults in many air cored reactors even though these turn to turn faults can quickly cause extensive damage to the reactor and often to other devices to which the reactor is connected.
The system to be described in the next section overcomes at least some of these limitations and is able to detect the smallest of faults in air core reactors, and furthermore has the decided advantage that the detector current is directly proportional to the severity of the fault that has occurred.